Numerous apparatus are known for the geometric measurement to align vehicle wheels, particularly those described in U.S. Pat. Nos. 3,865,492, 3,901,604, 4,319,838, 4,594,789, 4,854,702 and 5,018,853.
In these known apparatus, a source of light radiation secured to a wheel of the inspected vehicle illuminates a receiver secured to another wheel of the vehicle, which supplies a signal processed by the processing and application device of the signals to supply the desired angular values.
The usual arrangement of the geometric inspection apparatus of automotive vehicles is described particularly in U.S. Pat. No. 4,311,386: an assembly of six measurement casings is secured to the front and rear wheels of the inspected vehicle; a casing for each rear wheel and an assembly of two casings is rigidly interconnected to each front wheel. Each casing comprises a light source and a receiver sensitive to radiation emitted by the source, whereby the radiation source of a first casing shines toward the receiver of a second casing and the light source of this second casing shines toward the receiver of this first casing.
U.S. Pat. No. 4,311,386 thus discloses a process for measuring the parallelism of the wheels of the forward and rear trains of the automotive vehicles and an apparatus for practicing this process. To this end, there is disposed at the level of each front wheel a laser beam source, there is projected from the emitter associated with each front wheel a laser beam transversely in front of the vehicle in the direction of the other front wheel, said beam striking a receiver, there is projected from the same emitter associated with one of the front wheels a third laser beam toward a reflecting mirror disposed at the corresponding rear wheel and the third laser beam reflected from a third receiver associated with the same forward wheel is received, then for each receiver associated with a laser source, the distance between the point of impact of the laser beam and a zero reference is noted. The receiver associated with each of the laser beams projected from the emitter of a front wheel transversely in front of the vehicle is disposed in advance of the other front wheel. There is also projected, from the emitter of the other front wheel, a fourth laser beam toward a second reflecting mirror disposed at the corresponding rear wheel and there is received the fourth laser beam reflected from a fourth receiver associated with the same other front wheel and the distance between the point of impact of said laser beam and a zero reference is noted. The observed distances are introduced into the four receivers by a computer which supplies the array of the geometric characteristics of the vehicle, knowing the path and the wheel base of the vehicle.
However, in this improved method and apparatus, it is also necessary to align a wheel, for example a front wheel. To this end, the vehicle is raised and the forward wheel is caused to turn in the forward direction of the vehicle, so as to determine the misalignment curve of the wheel. The wheel is continued to be turned until the wheel will be in the position corresponding to median misalignment, then the vehicle is rested on the ground while maintaining the wheel in this position, which theoretically avoids influencing the parallelism when the wheel is in a position in which the median misalignment is horizontal and the maximum misalignment is vertical. One operates in an analogous fashion to effect the alignment of a rear wheel.
It will be noted, however, when the vehicle is rested on the wheels, that the action of the suspension devices, in particular the action of the suspension arm on the front wheel, gives rise to an undesired rotation of the wheel by an angle of the order of 10.degree. between the beginning of contact of the tire with the ground and the complete resting of the wheel on the ground. As a result there is an imperfection of measurement, because of the fact that the wheels resting on the ground are located in undesired angular positions, corresponding to a value of misalignment different from the mean misalignment.
An object of the invention is to improve the geometric measurement of vehicles, by supplying permanently the real value of the wheel angle, so as to effect a correction of the measurements as a function of the real misalignment of the wheel. By misalignment will be generally understood a curve or a sinusoidal signal varying as a function of the angular position of the wheel and representative of transverse deformations of the rim or the wheel.
The document DE-4 121 655 discloses an apparatus for measuring the angular position of the wheel, permitting deriving the value of misalignment of the wheel. Thanks to this device, it is possible to know the real position of the wheel, so as to be able to compensate the geometric measurements by a correction dependent on the phase of misalignment. To know the angular position of the wheel, there is provided mounted on an axis secured to a support fixed rigidly to the wheel, a toothed wheel, which drives a pinion coupled with a measurement converter supplying an electrical signal representative of the angular position of the wheel. The axle of the toothed wheel is disposed coaxially with the axis of the wheel, which gives rise to a measurement error corresponding to the relative play between said pinion and said toothed wheel.
A second object of the invention is to improve the measurement of the angular position of the wheel by means of an apparatus simultaneously integrating the conventional measurement means of the known values of parallelism and effecting corresponding corrections as a function of the misalignment of each wheel of the vehicle, in particular by correcting the values of parallelism obtained by means of apparatus with particular radiant light sources having an improved resolution and an improved measurement range, of simple design and economical manufacture, particularly using only low cost optical receivers of the linear type.
A third object of the invention is to provide an unchanging position of the device relative to the support secured to the wheel rim and to avoid any error arising from positioning errors resulting from successive mounting and demounting of the apparatus relative to the wheel rim.